1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine, a facsimile machine and a printer, and more particularly, to an image forming apparatus having bearing members for bearing toner, a transfer member and the transfer member with toner image which are conveyed to next process to produce an image formed transfer member.
2. Discussion of the Background
In recent years, electrophotographic image forming apparatus able to copy or print full color images have become practical to use. In these image forming apparatus, methods of transferring a full color image onto a transfer member are broadly classified into the following two types.
(a) Method using transfer drum: PA1 (b) Method using intermediate transfer element:
Each of Yellow (Y), magenta (M), cyan (C) and black (Bk) images are formed one by one on an image bearing member such as a photoconductive drum and transferred onto a transfer member secured on a transfer drum to form a full color image thereon.
Each of yellow, magenta, cyan and black images are formed one by one on an image bearing member such as a photoconductive element or drum and individually transferred onto an intermediate transfer element at a first transfer nip part to form a full color image thereon. The full color image formed on the intermediate transfer element is then transferred onto a transfer member at a sheet transfer nip part at the same time.
Between these two methods, the method using an intermediate transfer element is advantageous because of being able to form a full color image even on a thick transfer member and to form an image even on a tip end part of a transfer member on which an image cannot be formed by the method using a transfer drum because the leading edge part is clamped to secure the transfer member.
In the method using an intermediate transfer element, when a material having a medium resistance (volume resistivity of 10.sup.7 to 10.sup.13 .OMEGA..multidot.cm) is used as the intermediate transfer element, an undesirable image, so-called "transfer blurring" occurs in character images or line images after the toner images are transferred onto the intermediate transfer element.
Mechanism of the occurrence of the transfer blurring is considered as follows. By applying a predetermined transfer bias voltage to the intermediate transfer element at the first transfer nip part in which the photoconductive element contacts the intermediate transfer element to transfer the toner image from the photoconductive element to the intermediate transfer element, a transfer electric field is formed at the first transfer nip part, and thereby the toner image on the photoconductive element is electrostatically transferred onto the intermediate transfer element. If an insulator is utilized as the intermediate transfer element, the applied transfer bias voltage stays at the applied position (in this case, the first nip part) and has little if any effect on other parts of the intermediate transfer element.
However, in a case where a material having medium resistance is utilized as the intermediate transfer element, the applied transfer bias voltage affects the applied position as well as other parts of the intermediate transfer element. The transfer bias voltage affects both sides of the intermediate transfer element, namely, an upstream side and a downstream side in a moving direction of the intermediate transfer element, with respect to the voltage applied position.
When the transfer bias voltage affects an entrance of the first transfer nip part (a gap formed before the first transfer nip part in which the photoconductive element contacts the intermediate transfer element), an electric field is formed at the entrance of the first transfer nip part. This electric field causes so-called "pre-transfer" in which the toner image on the photoconductive element is transferred onto the intermediate transfer element before the photoconductive element contacts the intermediate transfer element at the first transfer nip part. Therefore, the toner image is pre-transferred onto a position of the intermediate transfer element slightly before the position to which the toner image should be transferred. As a result, the transferred toner image is wider and/or longer than the desired toner image, and the plural color toner images are superimposed on the intermediate transfer element not properly aligned. This is called "transfer blurring", and the final product is an undesirable image.
In the following discussion, a contacting part of the toner on the photoconductive drum and the intermediate transfer element (an intermediate transfer belt, for example), i.e., a first transfer area, is referred to as a first transfer nip part. Similarly, a contacting part of the toner on the intermediate transfer belt and the transfer member (sheet paper, for example), i.e., a second transfer area, is referred to as a second transfer nip part.
In the method using an intermediate transfer element, a drawback occurs in which undesirable images such as solid image scattering, and so-called "crow's claw mark," tend to occur in a toner image which is transferred to the transfer member from the intermediate transfer belt under low humidity environments. Solid image scattering is toner of a solid image scattered onto the background just before and after the transferred toner image in the feeding direction of the intermediate transfer element. The "crow's claw mark" is a streak-like high density toner image which looks like a crow's footmark observed in a half-tone image. It is believed that these phenomena occur due to occurrence of a difference between charge quantity on a front side of the transfer member on which the toner image is formed and charge quantity on a back side of the transfer member because the resistance of the transfer member is increased under low humidity environments.
More precisely, under low humidity environments, the charge quantity of the back side of the transfer member becomes greater than that of the front side of the transfer member after the transfer member is passed through the second transfer nip part. Therefore, the toner of the toner image on the transfer member scatters to the background just before and after the toner image along the electric field generated by the difference of the charge quantities of the front side and the back side of the transfer member, resulting in occurrence of solid image scattering. The "crow's claw mark" occurs by discharging due to the difference of the charge quantities of the front side and the back side of the transfer member.